Here's how it's derived:

* Ammonium ion (NH₄⁺): Ammonium is a polyatomic ion with a +1 charge.

* Iodide ion (I⁻): Iodide is a monatomic ion with a -1 charge.

To form a neutral compound, the charges must balance. Since the charges are equal and opposite, one ammonium ion and one iodide ion combine to form ammonium iodide (NH₄I).